* fix(caldav): use consistent UIDs and inject VTIMEZONE in iCalendar output
Two remaining CalDAV interop issues from #9485:
1. UID consistency: use the booking's canonical UID (event.uid) instead
of always generating a new random UUID. CalDAV servers use UID as the
event identifier — different UIDs cause duplicate calendar entries.
2. VTIMEZONE injection: the ics library generates UTC times with no
VTIMEZONE block. CalDAV servers like Fastmail read this as UTC and
send scheduling emails with wrong times. Per RFC 5545 §3.6.5,
DTSTART with TZID requires a matching VTIMEZONE component. We now
build a proper VTIMEZONE using binary-searched DST transitions for
the event's year, handling Northern/Southern hemisphere correctly.
* fix: use pre-transition offset for VTIMEZONE DTSTART per RFC 5545
The DTSTART in VTIMEZONE components must represent the local time
interpreted with the pre-transition offset (TZOFFSETFROM), not the
post-transition offset. For example, US Eastern spring forward DTSTART
should be 02:00 (EST), not 03:00 (EDT).
* Remove comments on UID handling in createEvent
Removed comments about UID handling for calendar events.
* Revise injectVTimezone documentation
Update injectVTimezone function documentation to clarify UTC handling.
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Co-authored-by: Anik Dhabal Babu <81948346+anikdhabal@users.noreply.github.com>
